Nickel and cobalt metal powders of high purity are generally produced by three techniques: (1) the carbonyl method based on the decomposition of nickel carbonyl gas under controlled conditions; (2) by the electrolytic method using a nickel or cobalt anode; and (3) by the hydrogen reduction of the metal from aqueous solutions thereof.
The carbonyl (CPM) and the electrolytically (ELM) produced metal powders tend to be more superior in high purity than the hydrogen reduced powders (HPM). The ASTM specification for nickel powder is compared below with some samples of ELM, CPM and HPM powders.
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__________________________________________________________________________ Elements, ppm S C O Fe Cu Pb As Sb Se Si Ca Co ASTM -- Spec. 100 300 NE** 200 200 50 50 50 NE** 50 NE** 1500 __________________________________________________________________________ PRO- ELM 5 50 100 20 20 &lt;10 &lt;10 &lt;10 &lt;5 &lt;10 &lt;15 &lt;200 DUCT CPM 5 .about.&lt;1000 100 15 10 &lt;10 &lt;10 &lt;10 -- 10 &lt;15 &lt;100 HPM .about. &lt;50 100 300 50 20 &lt;10 &lt;10 &lt;10 .ltorsim. 20 .about. &lt;100 &lt;15 &lt;1000 __________________________________________________________________________ **NE = Non-existent
The three commercial powders may vary in specification from producer to producer and may also vary according to a particular production run. This is especially the case in the production of the HPM product.
The HPM powder product, as stated earlier, is produced by the hydrogen reduction of nickel and cobalt from solutions. Such solutions as generally obtained by high pressure leaching of nickel lateritic ores or nickel mattes in sulfuric acid or ammoniacal solutions. The ores or mattes generally contain impurities (e.g. Si, Fe, Al, Cr, among others) which carry over into the leaching circuit.
Hydrometallurgical schemes for producing a high purity nickel (cobalt) powder are normally quite complex. An overall objective is to generate the purest possible streams of these metals before the final step of recovering these elements in the metallic form. The impurity content of trace elements in these streams should be below 1000 ppm for difficult to reduce elements like Ca, Mg, Zn; below 50 ppm for the elements which tend to coprecipitate with metals, such as Si, Fe, Al, Cr, As, Sb and less than 2 ppm for elements which tend to reduce to metallics as easily as nickel, for example, Cu, Pb and Se.
Methods for producing HPM nickel or cobalt powders of the foregoing type are disclosed in U.S. Pat. Nos. 2,753,257, 2,767,081, 2,767,082 and 2,767,083.
In a commercial operation, it is difficult to maintain the desired criteria as to high purity. Changes in compositions of the feeds, loss in a process control, external contamination (environmental dust, need to recover plant spillages, etc.), may more or less cause intolerable contamination of the final product. That is particularly the case with silicon.
An impurity level of a few parts per million and especially in the tens of ppm by weight (10 ppm=0.001%) can make the difference between salable and non-salable products, the latter generally leading to discounted price penalties which can lead to significant economic loss. This is particularly the case for silicon which is a common impurity in the production of hydrogen precipitated nickel or cobalt. For example, it should be noted that contamination of more than 0.5 parts Si per 10,000 parts by weight of metals is considered the limit allowable under the specification.
We have found that HPM produced powder can be significantly reduced in silicon content by treating the powder with a solution of an alkali metal hydroxide, such as a sodium hydroxide solution.
It is known in the preparation of Raney nickel to leach an alloy of nickel and silicon with sodium hydroxide in order to leach partially and selectively some of the contained silicon so as to produce a highly catalytic nickel at the surface of the alloy. The dissolution of the alloy is exothermic in that the sodium hydroxide reacts with silicon to form sodium silicate accompanied by evolution of heat.
One method of producing Raney nickel catalyst is disclosed in U.S. Pat. No. 2,583,619 which refers to a preferred alloy composition of 35 to 65% by weight of removable metal (e.g. silicon) and the balance nickel. The alloy surface is treated with an aqueous solution of sodium hydroxide (10% to 50% solution) at about 245.degree. F. to 250.degree. F. (118.degree. C. to 121.degree. C.) to remove selectively silicon and form a skeleton containing active nickel which is washed free of the treating solution with cold water.
The preparation of the Raney nickel catalyst is not the same as the present invention in that the nickel powder treated in the present invention is relatively pure and contains at least about 97% nickel plus cobalt, the silicon being generally present in the oxidic state as occluded silicates or as SiO.sub.2 due to external contamination. Normally, it is a fine two-phase mixture of the silicon impurity in the metallic product.